1. Field of Invention
This invention relates to an electroluminescenct display device and its manufacturing method.
2. Description of the Related Art
An electroluminescenct (referred to as EL hereinafter) display device with an EL element has been gathering attention as a display device substituting a CRT or an LCD. The development efforts for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching device for driving the EL device have been made accordingly.
FIG. 15 is a plan view showing a pixel of an EL display device of the prior arts. FIG. 16 shows a cross-sectional view of the device along with the B—B cross-sectional line. A TFT for driving an organic EL device is disposed near the crossing of a gate signal line 51 with a gate electrode 11 and a drain signal line 52. A drain of the TFT is connected to the drain signal line 52 and a gate of the TFT to the gate signal line 51. A source of the TFT is connected to an anode 61 of the EL device. A plurality of the pixels is disposed in a matrix configuration in an actual EL display device.
A display pixel 110 is formed by disposing the TFT and the organic EL device sequentially on a substrate 10, which is a glass substrate, a resin sybstrate, a conductive substrate, or a semiconductor substrate. When the conductive substrate or the semiconductor substrate is used as the substrate 10, an insulating film made of SiO2 or SiN should be disposed on the substrate first before forming the TFT.
First, the gate electrode 11 made of a metal with a high-melting point such as chrome (Cr) is disposed on the insulating substrate 10, and then a gate insulating film 12 and an active layer 13 made of p-Si film are sequentially disposed.
Ion doping is performed to a channel 13c of the active layer 13 located above the gate electrode 11 and to the areas located at both sides of the channel 13c using a stopper insulating film 14 as a mask. Furthermore, ion doping is performed with the both sides of the gate electrode 11 covered with resist, forming low density areas at the both sides of the gate electrode 11 and a source 13s and a drain 13d of high density areas each located outside of each of the low density areas. This configuration is known as the LDD (lightly doped drain).
Then an interlayer insulating film 15 is formed by sequentially disposing a SiO2 film, a SiN film, and a SiO2 film over the entire surface of the gate insulating film 12, the active layer 13 and the stopper insulating film 14. A drain electrode 16 is formed by filling a contact hole made corresponding to the drain 13d with a metal such as aluminum. Furthermore, a planarization insulating film 17 made of organic resin for planarization the surface is disposed on the entire surface.
A contact hole is formed in the planarization insulating film 17 at the location corresponding to the source 13s. Then, the anode 61, which also functions as a source electrode 18, made of ITO (Indium Tin Oxide) making a contact with the source 13s through the contact hole is formed on the planarization insulating film 17. The anode 61 is made of a transparent electrode such as ITO (Indium Tin Oxide). The EL device is disposed on the anode 61.
The organic EL device 60 has a conventional configuration, where the anode 61, a hole transportation layer 62 including a first hole transportation layer made of MTDATA (4, 4-bis(3-mathylphenylphenylamino) biphenyl) and a second hole transportation layer made of TPD (4, 4, 4-tris(3-methylphenylphenylamino) triphenylanine), an emissive layer 63 made of Bebq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivative, an electron transportation layer 64 made of Bebq2, and the cathode 65 made of either magnesium-indium alloy, aluminum, or aluminum alloy are disposed in this order.
The organic EL device 60 emits light by the electric current supplied through the driving TFT. In the organic El device 60, a hole injected from the anode 61 and an electron injected from the cathode 65 are recombined in the emissive layer and an exciton is formed by exciting an organic module forming the emissive layer 63. Light is emitted from the emissive layer 63 in a process of relaxation of the exciton and then released outside after going through the transparent anode 61 to the transparent insulating substrate 10.
The above-mentioned technology is described in, for example, Japanese Laid-Open Patent Publication No. Hei 2001-175200.
The plane area of the emissive layer 63 formed on the anode 61 defines the light-emitting area SO of the organic EL device 60, as shown in FIG. 16. Therefore, the larger the light-emitting area SO is, the brighter the organic EL device becomes. However, the larger light-emitting area SO usually requires the larger pixel area, which may cause the deterioration of fine display.
On the other hand, the life span of the organic EL device 60, that is, the duration till the brightness of the organic EL device 60 decreases to a certain point, is longer when the density of the current going through the organic EL device 60 is lower, as shown in FIG. 17. However, the brightness of the organic EL device 60 is low when the electric density is low.
Therefore, this invention is directed to the enlargement of the light-emitting area of the organic EL device as well as a higher brightness without making the individual pixel areas larger. Also, this invention is directed to the extension the life span of the organic EL device while keeping its brightness.